Metal-oxide-semiconductor field effect transistors (MOSFETs) are used in ultra-large scale integrated (ULSI) circuits, which are found in today's semiconductor integrated circuit (IC) chip products. The gate length of the MOSFET is continuously being scaled down for faster circuit speed, higher circuit density and increased functionality, and lower cost per unit function. As the gate length of the MOSFET is scaled into the sub-20 nm regime, the source and drain increasingly interact with the channel to substantially influence the channel potential. Hence, a transistor with a short gate length often suffers from problems related to the inability of the gate to substantially control the on/off states of the channel. Phenomena related to the reduced gate control of the channel potential are called short-channel effects.
Increased body doping concentration, reduced gate oxide thickness, and junction depths are some ways to suppress short-channel effects. However, for device scaling well into the sub-20 nm regime, the requirements for body-doping concentration, gate oxide thickness, and source/drain doping profiles become increasingly difficult to meet using conventional device structures based on bulk silicon substrates. Therefore, alternative device structures that offer better control of short-channel effects are being considered to enable the continued scaling down of transistor sizes.
A highly scalable device structure that offers superior control of short-channel effects is a wrap-around gate structure for a transistor (a.k.a., surround-gate or gate-all-around transistor structure). A wrap-around gate structure typically has a gate that surrounds or wraps around a channel region. This structure effectively improves the capacitance coupling between the gate and the channel, as compared to conventional bulk silicon substrate transistor structures, double-gate transistor structures, and triple-gate transistor structures. With the wrap-around gate structure, the gate gains significant influence on the channel potential, and therefore improves suppression of short-channel effects.